1. Field of the Invention
The present invention relates to boroxine compositions and, more particularly, to boroxine compositions which have fire and/or flame retardant properties. The compositions of the present invention are primarily, but not exclusively, intended to be used to cure epoxide or epoxide-type resins.
2. Description of the Related Art
At the present time, amines are generally used to cure epoxy resins. Such compounds do serve the purpose but only react slowly when mixed with the epoxy resin at room temperature. This time period, known as the pot life, is too long for many purposes. Another problem of using amines is that approximately stoichiometric quantities of the amine and epoxide must be used. The amine component of the cured product is highly flammable which is, of course, disadvantageous. Amine-epoxide compositions are not, therefore, suitable for fire-retardant applications unless large amounts of inorganic fillers such as Al(OH)3 or CaCO3 are used. However, these fillers render the composition opaque which makes them of little use in the glass industry.
Boroxines of the general formula: 
are known. As far as we are aware, the only boroxine which is commercially available is trimethoxy boroxine, that is to say, a compound of the above general formula in which each R represents a methyl group. Other boroxines have been made according to the literature but it appears that they have been made solely as an academic exercise and the literature gives no possible uses for such compounds.
Trimethoxy boroxine, hereafter referred to a TMB, has been used as for curing epoxy resins. The TMB opens the epoxide ring due to the presence of a lone pair of electrons on the oxygen atom of the epoxide group and this attacks one of the boron atoms in the cyclic boroxine structure. This has the advantage over amine-containing curing agents in that the reaction takes place at room temperature. However, there is a concomitant disadvantage which is that an ORxe2x88x92species leading to the production of an alcohol, such as methanol in the case of TMB, is simultaneously produced. Accordingly, a thermosetting resin is produced but highly volatile and inflammable methanol is also produced. This clearly increases the flammability characteristics of the cured material. If the methoxy groups in TMB were to be replaced by higher alkoxy groups, the boiling point of the alcohol produced is increased but the presence of a longer chain in the alcohol group effectively adds to the hydrocarbon content or xe2x80x9cfuelxe2x80x9d which is present. Accordingly, although the temperature at which the alcohol will ignite is increased, once this temperature is exceeded, the fire will burn more fiercely due to the presence of the additional fuel.
Obviously, if the amount of TMB present in the mixture is increased, the inorganic, that is to say, boroxine content of the mixture also increases. When such material is pyrolysed, it leaves a char which contains some glassy material so that the material is more fire-retardant. More recently, it has been attempted to add a catalyst inhibitor to the trimethoxy boroxine-epoxy resin mixture in an attempt to increase the TMB content so that the fire-retardancy can be increased without affecting the pot life. However to obtain significant improvements in fire retardancy, the levels of TMB needed cannot be attained due to constraints on the use of the inhibitor. One such inhibitor which has been tried is benzyl alcohol used in a ratio of 1 part benzyl alcohol to 3 partsTMB. Whilst this only slightly slows down the rate of curing, the disadvantage is that one is adding to the hydrocarbon content because benzyl alcohol is present as well as the by-product methanol which is produced. The flammability of the mixture is thus increased initially even though the end product of the pyrolysis, the char, contains more glass. It also did not prove possible to get enough TMB units into the polymer produced by the curing to produce a material having adequate heat resistance. In more technical terms, the glass transition temperature was too low as was the heat distortion temperature, this latter being the temperature at which a thermosetting polymer distorts.
When such a system is pyrolysed at approximately 800xc2x0 C., there is produced, as mentioned above, a charred material or char which contains a glassy material. The charred material also had a thin glassy coating on its exterior. A larger amount than was expected of charred material remained, and this is attributed to the formation of the glassy material. Normally, one would expect a comparatively small amount of charred material to remain because it burns at high temperature to yield carbon dioxide. The smaller than usual loss of charred material has had a greater strength than would normally be expected and this, again was attributed to the formation, in situ, of the glassy material. This is of importance, particularly if the polymeric material is being used as an interlayer between two sheets of glass. It will, of course, be appreciated that if only a small amount of char remains, this effectively means that the carbon has been xe2x80x9clostxe2x80x9d, that is to say, it has acted as an additional fuel and has been converted into carbon dioxide. Moreover, the loss of carbon will weaken a polymeric structure, particularly if it is being used as a fire-resistant interlayer.
Although inhibitors do have a beneficial effect insofar as fire retardancy is concerned, they do not provide an ideal solution because they are almost certain to be flammable compounds. Accordingly, we have directed our attention to compounds which, when pyrolysed, produce glass-forming precursors but which have low flammability tendencies and do not necessitate the use of inhibitors.
In our European Patent Specification No. EP 500317A, we disclose a boron compound which does not cure an epoxy resin but does provide a fire resistant composition in combination with the epoxy resin and trimethoxyboroxine as a curing agent therefor. In a preferred aspect of such invention, the boron compound which does not cure the epoxy resin is a boroester derived from a diol and boric acid. When such a compound pyrolises, the organic residues are burnt off and leaves a compound of the formula BXOY which is a glass precursor. The alkyl groups, generally methyl groups, do provide flammable components in the mixture. however, this additional flammability is very much less than that caused by the methanol by-product produced during the curing of the epoxy resin.
The present invention therefore seeks to provide a range of fire-retardant compositions which obviates, or at least minimises, the above disadvantages. More particularly, the present invention seeks to provide a range of compositions which have differing reaction times with epoxide groups but which, at the same time, have enhanced fire-retardant properties.
According to the present invention, there is provided a fire-retardant composition comprising a halo-substituted boroxine of the formula: 
in which R1, R2 and R3, which may be the same or different, each represent a lower alkyl group containing 2 to 5 carbon atoms or an aryl radical, at least one of R1, R2 and R3 being mono- or poly-substituted with at least one halogen substituent, in association with a compound containing an epoxide group which is to be cured, the boroxine being capable of curing the compound containing the epoxide group.
We have surprisingly found that, by using such boroxine compounds, the fire-retardant properties of the composition are greatly enhanced and that, by varying the length of the alkyl chain and/or by varying the location and number of halogen atoms, we can provide a range of curing compositions having differing pot lives. The reason for this is believed to be that when hydrocarbons per se are pyrolysed, they produce a large number of free radicals which recombine to form mixtures of hydrocarbons which, obviously, are highly flammable. By halogenating the alkyl chain or the aryl group, although the hydrocarbon free radicals are still produced, one or more hydrogen halides are also produced and these both dilute the hydrocarbon free radicals and do not support combustion. In other words, the flammability is substantially reduced. The substitution of organic compounds with halo-substituents is known to have fireproofing effects. However, when the material is, effectively, an organic compound having a substantial inorganic content, it cannot be presumed that the halo-substituents will have such an effect. One reason for this is that halogens normally act as fire-retardant materials in the gaseous phase ultimately producing hydrogen halide species in the gas phase whilst boron tends to act as a fire-retardant in the solid or condensed phase ultimately producing polyborates. There is no evidence in the literature that a combination of these compounds will have an added effect if used in combination, let alone the synergistic effect we have discovered.
Moreover, as will be well known in the art, just because trimethoxyboroxine is effective as a curing agent for epoxy resins, there is no guarantee whatsoever that an halogenated boroxine will cure the resin and, further, there is no indication that the use of such compounds will, in practice, provide improved fire-retardancy.
Another factor which could not be anticipated is that a range of substituted boroxines could produce compositions having a wide range of pot lives. Thus, for example, one can plot the viscosity change when a standard amount of an epoxide is reacted with a standard amount of a boroxine. We have found that if the two boroxines used are n-butoxyboroxine and 4-chlorobutoxyboroxine, the viscosity against time curve for the latter has a gradient which is four times greater than that of the former. This means, in effect, that the pot life of the latter is four times shorter than that of the former.
Thus, we have found that even with carbon chains of four or five atoms, not only do we still obtain a fire-retardant effect but also, even though the halogen atom or atoms are spaced at a considerable distance from the site at which the epoxide attacks the boroxine ring, that the curing rate is increased compared with trimethoxyboroxine itself.
These findings are surprising because one would normally expect that the replacement of an hydrogen atom by a chlorine atom would slow the reaction down due to steric hindrance. One would also expect that electronic inductive effects would also decrease as the substituent is located at a greater distance via several sigma bonds from the boroxine ring.
The alkyl chain may be straight or branched. We have found that branched chain compounds have a shorter pot life than the corresponding straight chain compounds, that is to say, they react faster with the epoxide. This is surprising because one would expect the branched chain compounds to be more sterically hindered than their straight chain counterparts and therefore to react more slowly. The preferred halogens are the fluoride and chloride and the alkyl chain may be mono- or poly-substituted, including perhalo-substituted.
The one or more halogen groups may, if at least one of R1, R2 and R3 is an aryl group, be directly substituted on the aryl moiety or may be provided on an alkyl side chain on the aryl moiety. It is desirable that the halogen is chlorine if it forms a substituent on an alkyl group and that it is fluorine if it forms a substituent on an alkyl or aralkyl group.
The preferred boroxine is selected from the group consisting of tri(2-chloroethoxy)boroxine, tri(2,2-dichloroethoxy)boroxine, tri(2,2,2-trichloroethoxy)boroxine, tri(3-chloro-1-propoxy)boroxine, tri(1,3-dichloro-2-propoxy)boroxine, tri(4-chloro-1-butoxy)boroxine, tri-(3-trifluoromethylbenzyloxy)boroxine, tri(2-fluorobenzyloxy)boroxine, tri(3-fluorobenzyloxy)boroxine, tri(4-fluorobenzyloxy)boroxine, tri(2,3,4,5,6-pentafluorobenzyloxy)boroxine, tri(2,2,3,3-tetrafluorobenzyloxy)boroxine, tri(1H,1H-pentafluoropropoxy)boroxine, tri(1H,1H,5H-octafluoropentoxy)boroxine and tri(1H,1H-heptafluorobutoxy)boroxine.
The preparation of certain boroxines used in the compositions of the present invention will be further described, by way of illustration only, with reference to the following non-limitative Examples: